Single wall carbon nanotubes (SWCNTs) are essentially graphene sheets rolled into hollow cylinders thereby resulting in tubules composed of sp2 hybridized carbon arranged in hexagons and pentagons, which have outer diameters between 0.4 nm and 10 nm. These SWCNTs are typically capped on each end with a hemispherical fullerene (buckyball) appropriately sized for the diameter of the SWCNT. However, these end caps may be removed via appropriate processing techniques leaving uncapped tubules. SWCNTs can exist as single tubules or in aggregated form typically referred to as ropes or bundles. These ropes or bundles may contain several or a few hundred SWCNTs aggregated through Van der Waals interactions forming triangular lattices where the tube-tube separation is approximately 3-4 Å. Ropes of SWCNTs may be composed of associated bundles of SWCNTs.
The inherent properties of SWCNTs make them attractive for use in many applications. SWCNTs can possess high (e.g. metallic conductivities) electronic conductivities, high thermal conductivities, high modulus and tensile strength, high aspect ratio and other unique properties. Further, SWCNTs may be metallic, semi-metallic, or semiconducting dependant on the geometrical arrangement of the carbon atoms and the physical dimensions of the SWCNT. To specify the size and conformation of single-wall carbon nanotubes, a system has been developed, described below, and is currently utilized. SWCNTs are described by an index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped into the form of a cylinder. When n=m e.g. (n, n), the resultant tube is said to be of the “arm-chair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. When m=0, the resultant tube is said to be of the “zig zag” or (n, 0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig zag pattern. Where n≠m and m≠0, the resulting tube has chirality. The electronic properties are dependent on the conformation, for example, arm-chair tubes are metallic and have extremely high electrical conductivity. Other tube types are semimetals or semi-conductors, depending on their conformation. SWCNTs have extremely high thermal conductivity and tensile strength irrespective of the chirality. The work functions of the metallic (approximately 4.7 eV) and semiconducting (approximately 5.1 eV) types of SWCNTs are different.
Similar to other forms of carbon allotropes (e.g. graphite, diamond) these SWCNTs are intractable and essentially insoluble in most solvents (organic and aqueous alike). Thus, SWCNTs have been extremely difficult to process for various uses. Several methods to make SWCNTs soluble in various solvents have been employed. One approach is to covalently functionalize the ends of the SWCNTs with either hydrophilic or hydrophobic moieties. A second approach is to add high levels of surfactant and/or dispersants (small molecule or polymeric) to help solubilize the SWCNTs.
Lavin et al. in U.S. Pat. No. 6,426,134 disclose a method to form polymer composites using SWCNTs. This method provides a means to melt extrude a SWCNT/polymer composite wherein at least one end of the SWCNT is chemically bonded to the polymer, where the polymer is selected from a linear or branched polyamide, polyester, polyimide, or polyurethane. This method does not provide opportunities for solvent based processing and is limited to melt extrusion which can limit opportunities for patterning or device making. The chemically bonded polymers identified typically have high molecular weights and could interfere with some material properties of the SWCNTs (e.g. electronic or thermal transport) via wrapping around the SWCNTs and preventing tube-tube contacts.
Connell et al in US Patent Application Publication 2003/0158323 A1 describes a method to produce polymer/SWCNT composites that are electronically conductive and transparent. The polymers (polyimides, copolyimides, polyamide acid, polyaryleneether, polymethylmethacrylate) and the SWCNTs or MWCNTs are mixed in organic solvents (DMF, N,N-dimethlacetamide, N-methyl-2-pyrrolidinone, toluene,) to cast films that have conductivities in the range of 10−5-10−12 S/cm with varying transmissions in the visible spectrum. Additionally, monomers of the resultant polymers may be mixed with SWCNTs in appropriate solvents and polymerized in the presence of these SWCNTs to result in composites with varying weight ratios. The conductivities achieved in these polymer composites are several orders of magnitude too low and not optimal for use in most electronic devices as electronic conductors or EMI shields. Additionally, the organic solvents used are toxic, costly and pose problems in processing. Moreover, the polymers used or polymerized are not conductive and can impede tube-tube contact further increasing the resistivity of the composite.
Kuper et al in Publication WO 03/060941A2 disclose compositions to make suspended carbon nanotubes. The compositions are composed of liquids and SWCNTs or MWCNTs with suitable surfactants (cetyl trimethylammonium bromide/chloride/iodide). The ratio by weight of surfactant to SWCNTs given in the examples range from 1.4-5.2. This method is problematic as it needs extremely large levels of surfactant to solubilize the SWCNTs. The surfactant is insulating and impedes conductivity of a film deposited from this composition. The surfactant may be washed from the film but this step adds complexity and may decrease efficiency in processing. Further, due to the structure formed in films deposited from such a composition, it would be very difficult to remove all the surfactant.
Papadaopoulos et al. in U.S. Pat. No. 5,576,162 describe an imaging element which comprises carbon nanofibers to be used primarily as an anti-static material within the imaging element. These materials may not provide the highly transparent and highly conductive (low sheet resistance, RS) layer that is necessary in many current electronic devices, especially displays.
Smalley et al in U.S. Pat. No. 6,645,455 disclose methods to chemically derivatize SWCNTs to facilitate solvation in various solvents. Primarily the various derivative groups (alkyl chains, acyl, thiols, aminos, aryls etc.) are added to the ends of the SWCNTs. The side-walls of the SWCNTs are functionalized primarily with fluorine groups resulting in fluorinated SWCNTs. The solubility limit of such “fluorotubes” in 2-propanol is approximately 0.1 mg/mL and in water or water/acetone mixtures the solubility is essentially zero. The fluorinated SWCNTs were subjected to further chemical reactions to yield methylated SWCNTs and these tubes have a low solubility in Chloroform but not other solvents. Such low concentrations are impractical and unusable for most deposition techniques useful in high quantity manufacturing. Further, such high liquid loads need extra drying considerations and can destroy patterned images due to intermixing from the excess solvent. In addition, the method discloses functionalization of the tubule ends with various functionalization groups (acyl, aryl, aralkyl, halogen, alkyl, amino, halogen, thiol) but the end functionalization alone may not be enough to produce viable dispersions via solubilization. Further, the side-wall functionalization is done with fluorine only, which gives limited solubility in alcohols, which can make manufacturing and product fabrication more difficult. Additionally, the fluorinated SWCNTs are insulators due to the fluorination and thereby are not useful for electronic devices especially as electronic conductors. Moreover, the chemical transformations needed to add these functional groups to the end points of the SWCNTs require additional processing steps and chemicals which can be hazardous and costly.
Smalley et al. in U.S. Pat. No. 6,683,783 disclose methods to purify SWCNT materials resulting in SWCNTs with lengths from 5-500 nm. Within this patent, formulations are disclosed that use 0.5 wt % of a surfactant, Triton X-100 to disperse 0.1 mg/mL of SWCNT in water. Such low concentrations are impractical and unusable for most deposition techniques useful in high quantity manufacturing. Further, such high liquid loads need extra drying considerations and can destroy patterned images due to intermixing from the excess solvent. In addition, the method discloses functionalization of the tubule ends with various functionalization groups (acyl, aryl, aralkyl, halogen, alkyl, amino, halogen, thiol) but the end functionalization alone may not be enough to produce viable dispersions via solubilization. Moreover, the chemical transformations needed to add these functional groups to the end points of the SWCNTs require additional processing steps and chemicals which can be hazardous and costly. Also, the patent discloses a composition of matter which is at least 99% by weight of single wall carbon molecules which obviously limits the amount of functionalization that can be put onto the SWCNTs thereby limiting its solubilization levels and processability.
Rinzler et al. in PCT Publication WO2004/009884 A1 disclose a method of forming SWCNT films on a porous membrane such that it achieves 200 ohms/square and at least 30% transmission at a wavelength of 3 um. This method is disadvantaged since it needs a porous membrane (e.g. polycarbonate or mixed cellulose ester) with a high volume of porosity with a plurality of sub-micron pores as a substrate which may lose a significant amount of the SWCNT dispersion through said pores thereby wasting a significant amount of material. Also, such membranes may not have the optical transparency required for many electronic devices such as displays. Further, the membrane is set within a vacuum filtration system which severely limits the processability of such a system and makes the roll-to-roll coating application of the SWCNT solution impossible. Moreover, the weight percent of the dispersion used to make the SWCNT film was 0.005 mg/mL in an aqueous solution. Such weight percents are impractical and unusable in most coating and deposition systems with such a high liquid load. Such high liquid loads make it virtually impossible to make patterned images due to solvent spreading and therefore image bleeding/destruction.
Blanchet-Fincher et al in Publication WO 02/080195A1 and in US 20040065970 A1 illustrate high conductivity compositions composed of polyaniline (PANI) and SWCNTs or MWCNTs and methods to deposit such compositions from a donor element onto a receiver substrate. The nitrogen base salt derivative of emeraldine polyaniline is mixed with SWCNTs in organic solvents (toluene, xylene, turpinol, aromatics) and cast into films with conductivity values of 62 S/cm (1 wt % SWCNT in PANI) and 44 S/cm (2 wt % SWCNT in PANI). These films alternatively may be produced as part of a multi-layer donor structure suitable for a material transfer system. The PANI/SWCNT composite are transferred from the donor sheet to a suitable receiver substrate in imagewise form. PANI is a highly colored conductive polymer thus resulting in a conductive composite with unsatisfactory transparency and color, thus it is not suitable for high transparency/high conductivity applications such as displays. Further, the conductivity values are not suitable for many electronic device applications. In addition, the compositions are made in organic solvents, which may require special handling for health and safety, making manufacturing difficult and expensive.
Hsu in WO 2004/029176 A1 disclose compositions for electronically conducting organic polymer/nanoparticle composites. Polyaniline (Ormecon) or PEDT (Baytron P) are mixed with Molybdenum nanowires or carbon nanotubes (8 nm diameter, 20 um length, 60 S/cm). The compositions disclosed in this invention are disadvantaged by marginal conductivity.
Arthur et al in PCT Publication WO 03/099709 A2 disclose methods for patterning carbon nanotubes coatings. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water (which may include viscosity modifying agents) are spray coated onto substrates. After application of the SWCNT coating, a binder is printed in imagewise fashion and cured. Alternatively, a photo-definable binder may be used to create the image using standard photolithographic processes. Materials not held to the substrate with binder are removed by washing. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water with viscosity modifying agents are gravure coated onto substrates. Dilute dispersions (10 to 100 ppm) of SWCNTs in isopropyl alcohol (IPA) and water are spray coated onto substrates. The coated films are then exposed through a mask to a high intensity light source in order to significantly alter the electronic properties of the SWCNTs. This step is followed by a binder coating. The dispersion concentrations used in these methods make it very difficult to produce images via direct deposition (inkjet etc.) techniques. Further, such high solvent loads due to the low solids dispersions create long process times and difficulties handling the excess solvent. In addition, these patterning methods are subtractive processes, which unnecessarily waste the SWCNT material via additional removal steps thereby incurring cost and process time. This application also discloses method to make conductive compositions and coatings from such compositions but it does not teach satisfactory methods nor compositions to execute such methods.
Transparent electronically-conductive layers (TCL) of metal oxides such as indium tin oxide (ITO), antimony doped tin oxide, and cadmium stannate (cadmium tin oxide) are commonly used in the manufacture of electrooptical display devices such as liquid crystal display devices (LCDs), electroluminescent display devices, photocells, solid-state image sensors and electrochromic windows or as components of these devices such as electromagnetic interference (EMI) shielding. They are also employed in resistive touch screens.
Devices such as flat-panel displays, typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods which involve high substrate temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a flexible substrate and organic electroconductive polymer layers as an electrode. Such plastic electronics allow low cost devices with new properties. Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous hopper or roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the “roll to roll” fabrication of electronic devices which are more flexible, lower cost, and lower weight.
Intrinsically conductive polymers have recently received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for TCL applications, some of these intrinsically conductive polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575, 5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472, 5,403,467, 5,443,944, 5,575,898, 4,987,042, and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550, 5,093,439, and 4,070,189) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conducting even at low humidity.
EP-A-440 957 describes a method for preparing polythiophene in an aqueous mixture by oxidative polymerization in the presence of a polyanion as a doping agent. In EP-A-686 662 it has been disclosed that highly conductive layers of polythiophene, coated from an aqueous coating solution, could be made by the addition of a di- or polyhydroxy and/or a carbonic acid, amide or lactam group containing compound in the coating solution of the polythiophene. Coated layers of organic electroconductive polymers can be patterned into electrode arrays using different methods. The known wet-etching microlithography technique is described in WO97/18944 and U.S. Pat. No. 5,976,274 wherein a positive or negative photoresist is applied on top of a coated layer of an organic electroconductive polymer, and after the steps of selectively exposing the photoresist to UV light, developing the photoresist, etching the electroconductive polymer layer and finally stripping the non-developed photoresist, a patterned layer is obtained. In U.S. Pat. No. 5,561,030 a similar method is used to form the pattern except that the pattern is formed in a continuous layer of prepolymer which is not yet conductive and that after washing the mask away the remaining prepolymer is rendered conductive by oxidation. Such methods that involve conventional lithographic techniques are cumbersome as they involve many steps and require the use of hazardous chemicals.
EP-A-615 256 describes a method to produce a pattern of a conductive polymer on a substrate that involves coating and drying a composition containing 3,4-ethylenedioxythiophene monomer, an oxidation agent, and a base; exposing the dried layer to UV radiation through a mask; and then heating. The UV exposed areas of the coating comprise non-conductive polymer and the unexposed areas comprise conductive polymer. The formation of a conductive polymer pattern in accordance with this method does not require the coating and patterning of a separate photoresist layer.
U.S. Pat. No. 6,045,977 describes a process for patterning conductive polyaniline layers containing a photobase generator. UV exposure of such layers produces a base that reduces the conductivity in the exposed areas.
EP-A-1 054 414 describes a method to pattern a conductive polymer layer by printing an electrode pattern onto said conductive polymer layer using a printing solution containing an oxidant selected from the group ClO−, BrO−, MnO4−, Cr2O7−2, S2O8−2, and H2O2. The areas of the conductive layer exposed to the oxidant solution are rendered nonconductive.
Research Disclosure, November 1998, page 1473 (disclosure no. 41548) describes various means to form patterns in conducting polymer, including photoablation wherein the selected areas are removed from the substrate by laser irradiation. Such photoablation processes are convenient, dry, one-step methods but the generation of debris may require a wet cleaning step and may contaminate the optics and mechanics of the laser device. Prior art methods involving removal of the electroconductive polymer to form the electrode pattern also induce a difference of the optical density between electroconductive and non-conductive areas of the patterned surface, which should be avoided.
Methods of patterning organic electroconductive polymer layers by image-wise heating by means of a laser have been disclosed in EP 1 079 397 A1. That method induces about a 10 to 1000 fold decrease in resistivity without substantially ablating or destroying the layer.
The application of electronically conductive polymers in display related device has been envisioned in the past. European Patent Application 1,172,831 describes a light transmissive substrate having a light transmissive conductive polymer coating for use in resistive touch screens. U.S. Pat. No. 5,738,934 describes touchscreen cover sheets having a conductive polymer coating.
U.S. Pat. Nos. 5,828,432 and 5,976,284 describe conductive polymer layers employed in liquid crystal display devices. The example conductive layers are highly conductive but typically have transparency of 60% or less.
Use of polythiophene as transparent field spreading layers in displays comprising polymer dispersed liquid crystals has been disclosed in U.S. Pat. Nos. 6,639,637 and 6,707,517. However, the polythiophene layers in these patents are non-conductive in nature.
Use of commercial polythiophene coated sheet such as Orgacon from Agfa has been suggested for manufacturing of thin film inorganic light emitting diode in U.S. Pat. No. 6,737,293. However, the transparency vs. surface electrical resistivity of such products may not be sufficient for some applications.
Use of conductive high molecular film for preventing the fringe field in the in-plane switching mode in liquid crystal display has been proposed in U.S. Pat. No. 5,959,708. However, the conductivity requirement for these films appears to be not very stringent. For example, in one embodiment (col. .5, lines 6-10) the high molecular film can be totally non-conductive. Moreover, U.S. Pat. No. 5,959,708 does not refer to any specification involving transmission characteristics of these films.
Use of transparent coating on glass substrates for cathode ray tubes using polythiophene and silicon oxide composites has been disclosed in U.S. Pat. No. 6,404,120. However, the method suggests in-situ polymerization of an ethylenedioxythiohene monomer on glass, baking it at an elevated temperature and subsequent washing with tetra ethyl orthosilicate. Such an involved process may be difficult to practice for roll-to-roll production of a wide flexible plastic substrate.
Use of in-situ polymerized polythiophene and polypyrrole has been proposed in U.S. Pat Appl. Pub. 2003/0008135 A1 as conductive films, for ITO replacement. As mentioned earlier, such processes are difficult to implement for roll-to-roll production of conductive coatings. In the same patent application, a comparative example was created using a dispersion of poly(3,4 ethylene dioxythiophene)/polystyrene sulfonic acid which resulted in inferior coating properties.
Addition of conductivity enhancing agents such as organic compounds with dihydroxy or polyhydroxy and/or carboxyl groups or amide groups or lactam groups are suggested for incorporation in polythiophene in U.S. Pat. No. 5,766,515. Recently, U.S. Pat. Appl. Pub. 2003/0193042 A1 claims further improvement in conductivity of polythiophene through the addition of a substantial quantity of organic compounds such as phenols. But, health and safety concerns will dictate special precautionary measures, which may need to be taken, for the introduction of such hazardous compounds to a typical web manufacturing and coating site, thus possibly adding cost to the final product.
In another recent publication titled “Hydroxylated secondary dopants for surface resistance enhancement in transparent poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) thin films” by B. D. Martin, N. Nikolov, S. K. Pollack, A. Saprigin, R. Shashidhar, F. Zhang and P. A. Heiney, published in Synthetic Metals, vol. 142 (2004), p. 187-193, it was stated that the addition of small hydroxylated secondary dopants could greatly decrease the surface resistance of polythiophene films without reducing film transparency.
Devices such as resistive touch screens also employ a substrate provided with transparent conductors. Typically substrates are formed from rigid glass and a coating of ITO forms the transparent conductor. However, in this application, a flexible transparent cover sheet coated with a transparent conductor overlies the substrate and is separated from the substrate by spacer dots. The flexible cover sheet is deformed in operation by a stylus or finger to bring the two transparent conductive layers into contact. ITO is also commonly employed as the transparent conductor on the flexible transparent cover sheet. However, ITO tends to crack under stress and its conductivity altered. More flexible conductive polymers have also been considered for this application, but these conductive polymers are softer than ITO and tend to degrade from repeated contacts.
As indicated herein above, the art discloses a wide variety of electronically conductive TCL compositions that can be incorporated in displays and touch screens. Although application of electronically conductive polymers in display and touch screen related devices has been contemplated in the past, the stringent requirements of high transparency, low surface electrical resistivity, flexibility, and robustness under repeated contact demanded by modern display devices or touch screens is extremely difficult to attain with electronically conductive polymers. Thus, there is still a critical need in the art for transparent conductors that can be coated roll-to-roll on a wide variety of substrates under typical manufacturing conditions using environmentally desirable components. In addition to providing superior electrode performance, the TCL layers also must be highly transparent, must be patternable, must resist the effects of humidity change, and be manufacturable at a reasonable cost.